APPLICATION
-include
  -abc.h
  -xyz.h
  -pdq.h
ABC
  -abc.c
  -abc_appSpec.h
  -abc_map.h
  -abc_internal.c
  -abc_internal.h
-XYZ
  -xyz.c
  -xyz_appSpec.h
-PDQ
  -pdq.c
  -pdq_appSpec.h

Example 2: Where module source files would reside in a typical source directory for divided file structures.